The present invention concerns an arrangement for applying a coating mix, a surface sizing agent, or some other treating agent onto the surface of a moving base material web in the manufacture of coated paper or board.
Different kinds of treating agents can be applied onto the surface of a base material web in order to improve the printability properties of paper or board. The most common agents used include solid mineral agents, starch-based sizing agents, latex agents and various kinds of mixes. Printability is mainly improved by different mixes of solid substances applied onto the web surface in the form of coating slips mixed with water. Other agents can be used to act on e.g. the strength and water resistance of the base web. In addition, different kinds of special grades of paper are manufactured whose surface is coated with, e.g. micro capsules in order to produce a self-copying surface.
Different coating or application methods are used for the different treating agents. Short-dwell application constitutes a versatile coat application method wherein the coating mix is applied onto the web from a chamber restricted in the travel direction of the web by a front wall and a doctor blade or rod. The coating mix is transferred onto the web from the chamber and is smoothed immediately after application by means of the doctor device limiting off the chamber. The coating mix is fed into the application chamber through a feed slit in the bottom of the chamber. The supply of coating mix should be as even as possible in order to obtain maximal steadiness of the pressure of the coating mix on the web limiting off the chamber. Part of the coating mix is led over the front wall of the chamber into the incoming direction of the web. Outflow is used in seeking to achieve an even web damping line and to prevent the air brought along by the moving web from entering the application chamber.
Short-dwell application has a number of considerable benefits which make it well suited for the manufacture of certain grades of paper. In short-dwell application little damping of the base web occurs because the pressure on the application area is short and the pressure in the chamber is small. Thus, the application process imposes little strain on the web and, owing to the low degree of damping, better web strength is retained after application. As the smoothing out of the coat almost coincides with the application step, the coating mix does not have time to set onto the web surface and the effort needed for doctoring is smaller; hence, even this contributes to the smaller strain imposed on the web. Short-dwell application arrangements achieve small amounts of coating mix more easily than many other types of application devices, wherefore they are well suited for the manufacture of light weight coated (LWC) paper grades, and, due to the small strain the web is exposed to, for grades having a low grammage or strength value.
A moving web which limits off the application chamber gives rise to turbulence in the application chamber, and the flow rate of the coating mix at the surface of the web is equal to web velocity but is reduced towards the inside of the chamber. The turbulence is further affected by the feed flow of the coating mix. This turbulence is one of the reasons, according to present understanding the most important one, for the problem with short-dwell application, i.e. striation of the coat, or the occurrence of occasional streaks. Streaks in the web occur particularly at high velocities and in the case of large amounts of coating mix, and are thought to be a consequence of the unstability of the damping line of the web and the unstable flows in the application chamber. The term damping line is used to refer to the cross-machine line at which the web meets the coating mix. Usually this line runs essentially at the front wall of the application chamber, slightly into the incoming direction of the web. An unstable, serrated damping line and unstable flows in the cross direction of the web give rise to local differences in the thickness of the mix at the doctor blade, and thus in the impulse force exerted on the doctor blade.
Streaks in coated paper restrict the use of current short-dwell coating apparatuses at high velocities, because turbulence and streakiness are naturally the greater, the higher the velocity of the web when it passes the application chamber.
Two separate turbulences are generated in the chamber, the one in the immediate vicinity of the doctor blade close to the tip thereof being smaller than the turbulence formed in the middle of the chamber by the flow returning from the doctor blade and turning again into the web direction. The intensity of the greater turbulence is further enhanced by the movement of the coating mix supply flow from the lower edge of the chamber. The role played by these two turbulences in the appearance of streaks is not explicitly known but possibly one of the turbulences gives rise to changes in pressure in the chamber. Changes in pressure alter the force balance at the tip of the blade whereby the doctor force and the amount of coating mix remaining on the web undergo a local and short-duration change.
The occurrence of streaks may take a positive or a negative form, i.e. the amount of coating mix may increase or decrease locally with regard to the desired amount of coating mix. The occurrence of streaks resulting from turbulences in the application chamber is rapidly increased when the velocity of the web to be coated exceeds 1000 m/min.
The present invention aims at providing a short-dwell applicator which can be used to at least reduce the occurrence of streaks and which can be run at speeds exceeding those in use today.
The invention is based on a screen plate being adapted in the application chamber, dividing the application chamber such that the feed orifice or orifices for the treating agent are on one side of the screen plate and the doctor is on the other side of said plate.
According to an advantageous embodiment of the invention, an adjustable spout is adapted at the front edge of the application chamber, said spout being movable in the travel direction of the web in relation to the doctor such that the surface area of the part of the application chamber which is in contact with the web can be adjusted, i.e. the length of the application chamber can be altered in the travel direction of the web. At the same time it is possible to affect the dwell time of the application as well as the length of the pressure impulse.
The invention achieves considerable benefits.
The screen plate can be used to considerably reduce or even eliminate the occurrence of streaks, which makes it possible to increase web speed without giving rise to any streaks. The screen plate prevents turbulence caused by the return flow from the doctor blade or rod toward the lower part of the application chamber by decelerating the flow and by levelling down any local variations in speed and pressure. With the making even of the pressure in the application chamber and the velocity distribution of the flows, the force exerted on the doctor means is also even, thus resulting in a coating layer of homogeneous thickness. The invention renders it easier to achieve greater amounts of coating mix, which gives the short-dwell applicator a broader range of possible uses. The coating mix is usually applied in an amount of 5 to 13 g/m2.
The front edge of the chamber can be used to optimize the operating conditions during runs in accordance with velocity, paper grade, and treating agent, and during the sealing of the web the chamber orifice can be set such that it is very small, thus preventing any wetting of the seal. If the spout is adapted such that it is adjustable even in the lateral direction of the web by fitting it with profiling equipment, the wetting line between the web and the mix can be altered, thus affecting the coating mix profile to remain on the web. The screen can also be used to close the side of the application chamber on the side of the supply orifice altogether such that any paper chaff possible coming off the web is prevented from blocking the supply orifice. The formation of paper chaff is at its maximum during breaks but some fibres always come off the web when it is treated.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.